Deposition pressure-driven evolution of composition, structure, optical, and electrochemical properties in LiNixMnyCozO2 thin films

The development of high-performance LiNi_xMn_yCo_zO₂ (NMC) thin-film cathodes for advanced lithium-ion micro-batteries requires precise control over deposition conditions to optimize structural and electrochemical properties. This study investigates the influence of Ar working pressure (4, 8, 12, 16, and 20 mTorr) during RF magnetron sputtering on the physicochemical characteristics of NMC thin-film, focusing on potential effects from residual gas contamination. Structural analysis reveals that intermediate pressures (8 mTorr) enhance crystallinity, reduce lattice spacing (4.68 Å), and induce compressive stress (−0.811 GPa). SEM demonstrated pressure-dependent morphological evolution, with higher pressures (20 mTorr) yielding thinner films (66.6 nm) and finer grains (21.4 nm) due to reduced adatom mobility. XPS highlighted elemental modulation, showing lithium depletion (32.57–18.04 %) at elevated pressures, alongside increased Mn²⁺/Mn³⁺ and Co²⁺/Co³⁺ oxidation states, indicative of oxygen vacancy generation. Optical bandgap narrowing (3.22–2.91 eV) with pressure highlights defect-induced electronic modifications. Electrochemical testing of the 8 mTorr film reveals an initial discharge capacity of 65.21 mAh/g, but with rapid capacity fade (9.29 % retention after 60 cycles), accompanied by charge transfer resistance (R_ct = 779.30 Ω) and microstructural degradation. These findings serve as proof-of-concept data, highlighting the influence of sputtering pressure on film properties and guiding future optimization for integrated microbattery development.